Method for generating a gas which may be used for startup and shutdown of a fuel cell

ABSTRACT

The present invention provides a method for generating a gas that may be used for startup and shutdown of a fuel cell. In a non-limiting embodiment, the method may include generating a nitrogen-rich stream; merging the nitrogen-rich stream with a hydrocarbon fuel stream into a feed mixture stream; and catalytically converting the feed mixture into a reducing gas.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. PatentApplication No. 12/554,039, entitled Method For Generating A Gas WhichMay Be Used For Startup And Shutdown Of A Fuel Cell, filed on Sep. 4,2009, and is related to U.S. Patent Application No. 12/554,460, entitledApparatus For Generating A Gas Which May Be Used For Startup AndShutdown Of A Fuel Cell, filed on Sep. 4, 2009, each of which isincorporated herein by reference.

GOVERNMENT RIGHTS IN PATENT

This invention was made with Government support under DE-FC26-06NT42809awarded by DOE. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to fuel cell systems, and moreparticularly, to methods for generating a gas which may be used forstartup and shutdown of a fuel cell.

BACKGROUND

Fuel cell systems, such as fuel cell based power plants and mobile fuelcell based power generation equipment, generate electrical power viaelectrochemical reactions, and are coming into greater use because theexhaust byproducts are typically cleaner than traditional power plants,and because fuel cells may generate electricity more efficiently thantraditional power plants. Fuel cell systems often employ stacks ofindividual fuel cells, each fuel cell typically including an anode, acathode, and an electrolyte positioned between the anode and thecathode. The electrical load is coupled to the anode and the cathode.The anode and cathode are electrically conductive and permeable to therequisite gases, such as hydrogen and oxygen, respectively. In a solidoxide fuel cell (SOFC), the electrolyte is configured to pass oxygenions, and has little or no electrical conductivity so as to prevent thepassage of free electrons from the cathode to the anode. In order forthe electrochemical reactions to take place efficiently, some fuel cellsare operated at elevated temperatures, e.g., with anode, cathode andelectrolyte temperatures in the vicinity of 700° C. to 1000° C. orgreater for an SOFC.

During normal operation, a synthesis gas is supplied to the anode, andan oxidant, such as air, is supplied to the cathode. Some fuel cellsystems include an internal reformer that catalytically reforms the fuelinto a synthesis gas (syngas) by use of an oxidant. The fuel may be aconventional fuel, such as natural gas, gasoline, diesel fuel, or analternative fuel, such as bio-gas, etc. The synthesis gas typicallyincludes hydrogen (H₂), which is a gas frequently used in fuel cells ofmany types. The synthesis gas may contain other gases suitable as afuel, such as carbon monoxide (CO), which serves as a reactant for somefuel cell types, e.g., SOFC fuel cells, although carbon monoxide may bedetrimental to other fuel cell types, such as PEM (proton exchangemembrane) fuel cells. In addition, the synthesis gas typically includesother reformer byproducts, such as water vapor and other gases, e.g.,nitrogen and carbon dioxide (CO₂), methane (typically 1%), as well astrace amounts of higher hydrocarbon slip, such as ethane.

In any event, the synthesis gas is oxidized in an electrochemicalreaction in the anode with oxygen ions received from the cathode viamigration through the electrolyte. The reaction creates water vapor andelectricity in the form of free electrons in the anode that are used topower the electrical load. The oxygen ions are created via a reductionof the cathode oxidant using the electrons returning from the electricalload into the cathode.

Once the fuel cell is started, internal processes maintain the requiredtemperature for operation. However, in order to start the fuel cell, theprimary fuel cell system components must be heated, and some fuel cellsystem components must be protected from damage during the startup. Forexample, the anode may be subjected to oxidation damage in the presenceof oxygen at temperatures below the normal operating temperature in theabsence of the synthesis gas. Also, the reformer may require a specificchemistry in addition to heat, in order to start the catalytic reactionsthat generate the synthesis gas. Further, the startup of the fuel cellsystem should be accomplished in a safe manner, e.g., so as to prevent aflammable mixture from forming during the starting process. Stillfurther, it is desirable to purge the fuel cell with a non-explosive andnon-oxidizing gas during the initial stage of startup.

What is needed in the art is an improved apparatus and method forstartup and shutdown of a fuel cell.

SUMMARY

The present invention provides an apparatus and method that may be usedfor startup and shutdown of a fuel cell. For example, embodiments of thepresent invention may employ a nitrogen generator to generate anitrogen-rich stream, e.g., using a nitrogen separation membrane, thatmay be used to purge one or more auxiliary subsystem components or othercomponents of a fuel cell power plant.

In addition, the same and/or other embodiments of the present inventionmay include generating a low oxygen content oxidant, combining theoxidant with fuel to yield a feed mixture, and then catalyticallyconverting the feed mixture to a reducing gas.

Also, the same and/or other embodiments of the present invention mayinclude varying the oxygen content of the oxidant and also varying theoxidant/fuel ratio of the feed mixture in order to maintain the reducinggas at a desired temperature, e.g., a reaction temperature or atemperature downstream of the reactor.

Further, the same and/or different embodiments of the present inventionmay include controlling the oxygen content of the oxidant and alsocontrolling the oxidant/fuel ratio of the feed mixture in order tomaintain a desired catalyst exit temperature, while providing a desiredreducing strength of the reducing gas, e.g., by varying the combustiblescontent of the reducing gas, while providing a desired flow rate ofreducing gas.

Still further, the same and/or different embodiments of the presentinvention may include controlling the oxygen content of the oxidant andalso controlling the oxidant/fuel ratio of the feed mixture in order tomaintain a desired catalyst exit temperature, while providing a desiredreducing strength of the reducing gas, e.g., by varying the combustiblescontent of the reducing gas, while varying the reducing gas flow.

Yet further, the same and/or different embodiments of the presentinvention may include controlling the oxygen content of the oxidant andalso controlling the oxidant/fuel ratio of the feed mixture in order tomaintain a desired reducing gas catalyst exit temperature, whilevarying, e.g., changing, the reducing strength of the reducing gas,e.g., by varying the combustibles content of the reducing gas.

Still yet further, the same and/or different embodiments of the presentinvention may include maintaining a temperature, e.g., of a heatingdevice, at or above the catalyst auto-ignition temperature of the feedmixture in order to reduce the amount of time required to beginproducing reducing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawings,wherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 schematically depicts a fuel cell system in accordance with anembodiment of the present invention.

FIG. 2 schematically depicts the fuel cell system of FIG. 1 in greaterdetail, including a reducing gas generator in accordance with anembodiment of the present invention.

FIGS. 3A-3D are a flowchart depicting a method for startup and shutdownof a fuel cell using a reducing gas generator in accordance with anembodiment of the present invention.

FIG. 4 is a plot depicting catalytic conversion parameters in acatalytic reactor of a reducing gas generator in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

For purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings, and specific language will be used to describe the same.It will nonetheless be understood that no limitation of the scope of theinvention is intended by the illustration and description of certainembodiments of the invention. In addition, any alterations and/ormodifications of the illustrated and/or described embodiment(s) arecontemplated as being within the scope of the present invention.Further, any other applications of the principles of the invention, asillustrated and/or described herein, as would normally occur to oneskilled in the art to which the invention pertains, are contemplated asbeing within the scope of the present invention.

Referring now to the figures, and in particular, FIG. 1, a schematic ofa fuel cell system 10 in accordance with an embodiment of the presentinvention is depicted. Fuel cell system 10 includes one or more of afuel cell 12, and includes a reducing gas generator 14. Fuel cell system10 is configured to provide power to an electrical load 16, e.g., viaelectrical power lines 18. In the present embodiment, fuel cell 12 is asolid oxide fuel cell (SOFC), although it will be understood that thepresent invention is equally applicable to other types of fuel cells,such as alkali fuel cells, molten-carbonate fuel cells (MCFC),phosphoric acid fuel cells (PAFC), and proton exchange membrane (PEM)fuel cells. In the present embodiment, fuel cell system 10 is suitable,but not limited to, use in a fuel cell turbine hybrid system wherehigh-pressure feed streams are employed.

Reducing gas generator 14 of the present embodiment is configured togenerate a reducing gas having a combustibles content (which isprimarily hydrogen —H₂ and carbon monoxide —CO) that may be variedwithin a compositional range of approximately 3% combustibles content toapproximately 45% combustibles content. In other embodiments, differentcompositional ranges may be employed, for example, a range ofapproximately 2% combustibles content to approximately 50% combustiblescontent in some embodiments, and approximately 1% combustibles contentto approximately 60% combustibles content in other embodiments. As setforth below, reducing gas generator 14 of the present embodiment istailored to yield a start gas in the form of a reducing gas having aprimary function of protecting the anode of fuel cell 12 from oxidationduring startup of fuel cell 12, e.g., during system heat-up prior topower generation. As power generation is started, the reducing gas istransitioned off.

In the embodiment of FIG. 1, various features, components andinterrelationships therebetween of aspects of an embodiment of thepresent invention are depicted. However, the present invention is notlimited to the particular embodiment of FIG. 1 and the components,features and interrelationships therebetween as are illustrated in FIG.1 and described herein. For example, other embodiments encompassed bythe present invention, the present invention being manifested by theprinciples explicitly and implicitly described herein via the presentFigures and Detailed Description and set forth in the Claims, mayinclude a greater or lesser number of components, features and/orinterrelationships therebetween, and/or may employ different componentsand/or features having the same and/or different nature and/orinterrelationships therebetween, which may be employed for performingsimilar and/or different functions relative to those illustrated in FIG.1 and described herein.

Referring now FIG. 2, fuel cell 12 and reducing gas generator 14 aredescribed in greater detail. Fuel cell 12 includes at least one each ofan anode 20, an electrolyte 22, a cathode 24, and a reformer 26. Anode20, electrolyte 22 and cathode 24 are considered part of fuel cell 12.Reformer 26 is an internal steam reformer that receives steam as aconstituent of a recycled fuel cell product gas stream, and heat foroperation from fuel cell 12 electro chemical reactions. Reducing gasgenerator 14 is not a part of fuel cell 12, but rather, is configuredfor generating gases for use in starting up and shutting down fuel cell12.

Anode 20 is electrically coupled to electrical load 16 via electricalpower line 18, and cathode 24 is also electrically coupled to electricalload 16 via the other electrical power line 18. Electrolyte 22 isdisposed between anode 20 and cathode 24. Anode 20 and cathode 24 areelectrically conductive, and are permeable to oxygen, e.g., oxygen ions.Electrolyte 22 is configured to pass oxygen ions, and has little or noelectrical conductivity, e.g., so as to prevent the passage of freeelectrons from cathode 24 to anode 20.

Reformer 26 is coupled to anode 20, and is configured to receive a fueland an oxidant and to reform the fuel/oxidant mixture into a synthesisgas (syngas) consisting primarily of hydrogen (H₂), carbon monoxide(CO), as well as other reformer by-products, such as water vapor in theform of steam, and other gases, e.g., nitrogen and carbon-dioxide (CO₂),methane slip (CH₄), as well as trace amounts of hydrocarbon slip. In thepresent embodiment, the oxidant employed by fuel cell 12 during normaloperations, i.e., in power production mode to supply electrical power toelectrical load 16, is air, and the fuel is natural gas, although itwill be understood that other oxidants and/or fuels may be employedwithout departing from the scope of the present invention.

The synthesis gas is oxidized in an electro-chemical reaction in anode20 with oxygen ions received from cathode 24 via migration throughelectrolyte 22. The electro-chemical reaction creates water vapor andelectricity in a form of free electrons on the anode that are used topower electrical load 16. The oxygen ions are created via a reduction ofthe cathode oxidant using the electrons returning from electrical load16 into cathode 24.

Once fuel cell 12 is started, internal processes maintain the requiredtemperature for normal power generating operations. However, in order tostart the fuel cell, the primary fuel cell system components must beheated, including anode 20, electrolyte 22, cathode 24 and reformer 26.

In addition, some fuel cell 12 components may be protected from damageduring the start-up, e.g., due to oxidation. For example, anode 20 maybe subjected to oxidative damage in the presence of oxygen attemperatures above ambient but below the normal operating temperature offuel cell 12 in the absence of the synthesis gas. Also, reformer 26 mayneed a specific chemistry, e.g. H₂O in the form of steam in addition tothe heat provided during start-up of fuel cell 12, in order to start thecatalytic reactions that generate the synthesis gas. Further, it isdesirable that fuel cell 12 be started in a safe manner, e.g., so as toprevent a combustible mixture from forming during the starting process.Thus, it may be desirable to purge anode 20 with a nonflammable reducinggas during the initial startup as the temperature of anode 20 increased.In one aspect, a characteristic of reducing gas generator 14 is that thereducing gas may be made sufficiently dilute in combustibles to preventthe potential formation of a flammable (i.e., potentially explosive)mixture upon mixing with air. This may be desirable during the lowtemperature portion of heat-up of fuel cell 12 where any combustiblesmixing with air are below auto-ignition temperature, and therefore, canpotentially build up to form dangerous quantities of potentiallypressurized flammable gases within the vessel that contains fuel cell12.

The reducing gas strength for protecting anode 20 of fuel cell 12 fromoxygen migration can be quite high, e.g., up to 45% combustibles contentin the present embodiment, up to 50% in other embodiments, and up to 60%combustibles content in still other embodiments. Mechanisms that causethe migration of oxygen through electrolyte 22 to the anode 20 side ofthe fuel cell 12 are often temperature dependent and include oxygenpermeation through electrolyte 22 or oxygen transfer induced by shortcircuit currents. Also, physical leakage mechanisms may become worsewith temperature as materials differentially expand. Thus, the abilityof reducing gas generator 14 to increase combustibles content at highfuel cell 12 temperatures during startup may be particularly useful inprotecting anode 20 from oxidation damage.

From a safety perspective, it may be possible to step to a greaterreducing strength at higher temperatures during fuel cell 12 startup,since the possibility of mixing the reducing gas with a pressurizedvolume of air to form an combustible mixture in or near fuel cell 12 isreduced if the reducing gas is above auto-ignition temperature, becausethe reducing gas would tend to immediately burn upon mixing with air. Inaddition, this may prevent build-up of a flammable mixture that canpotentially deflagrate if the mixture were to suddenly come in contactwith an ignition source, since any such mixture would tend to burnimmediately when above the auto-ignition temperature, rather than buildup a large quantity of the mixture.

Thus, in some embodiments, it may be desirable to operate reducing gasgenerator 14 in a manner by which the reducing gas is initially weaklyreducing and well below the flammability limit, e.g., 3% combustiblescontent in the present embodiment, although other values may beemployed, for example, 2% combustibles content in some embodiments and1% combustibles content or less in other embodiments. In still otherembodiments, the combustibles content may be greater than 3%. Thecombustibles content may subsequently be changed to a strongly reducing(i.e., higher combustibles) condition (higher reducing strength) whentemperature conditions in fuel cell 12, e.g., anode 20, are high enoughto ensure that the reducing gas is far above its lower flammabilitylimit. For example, the strongly reducing condition may be up to 45%combustibles content in the present embodiment, up to 50% combustiblescontent in other embodiments, and up to 60% combustibles content orgreater in yet other embodiments, depending upon the conditions in fuelcell 12. The increased energy input to the system with a strongerreducing gas may be offset by decreasing fuel flow to the fuel cellpower plant's Off-Gas Burner for such plants so equipped.

Accordingly, embodiments of the present invention may employ reducinggas generator 14 to generate a purging gas to purge fuel cell 12 ofoxidants, in particular, cathode 24, as well as to generate a safe gas,i.e., a weak reducing gas having a relatively low level of combustibles.

In addition, embodiments of the present invention may also employreducing gas generator 14 to produce a variable-reducing-strengthreducing gas. The reducing gas composition provided by reducing gasgenerator 14 may also be configured to contain adequate steam toinitiate the operation of the internal reformer 26 as the normal fuelcell 12 fuel stream flow, e.g., natural gas, is started. Accordingly,the reducing gas supplied to fuel cell 12 from reducing gas generator 14may be considered a transition gas as power production by fuel cell 12is ramped up. Additionally, reducing gas generator 14 of the presentembodiment may be capable of rapid start-up, e.g., for protecting anode20 in the event of emergency fuel cell 12 shutdown events, for example,by maintaining certain elements of reducing gas generator 14 at elevatedtemperatures in order to speed up initiation of the catalytic reactionsthat yield the reducing gas.

In the present embodiment, as illustrated in FIG. 2, reducing gasgenerator 14 includes a fuel system 28, an oxidant system 30, a mergingchamber 32, and a catalytic reactor 34 having a catalyst 36. In thepresent embodiment, the outputs of fuel system 28 and oxidant system 30are combined in merging chamber 32 and directed to fuel cell 12 viacatalytic reactor 34 to selectively provide purging gas, safe gas, andvariable strength reducing gas to anode 20 and reformer 26.

In the embodiment depicted in FIG. 2, various features, components andinterrelationships therebetween of aspects of an embodiment of thepresent invention are depicted. However, the present invention is notlimited to the particular embodiment of FIG. 2 and the components,features and interrelationships therebetween as are illustrated in FIG.2 and described herein. For example, other embodiments encompassed bythe present invention, the present invention being manifested by theprinciples explicitly and implicitly described herein via the presentFigures and Detailed Description and set forth in the Claims, mayinclude a greater or lesser number of components, features and/orinterrelationships therebetween, and/or may employ different componentsand/or features having the same and/or different nature and/orinterrelationships therebetween, which may be employed for performingsimilar and/or different functions relative to those illustrated in FIG.2 and described herein.

In any event, in the embodiment of FIG. 2, fuel system 28 includes afuel input 38, a pressure regulator 40, a sulfur capture sorbent 42, afuel flow controller 44, and a variable position/output fuel controlvalve 46. Fuel input 38 is configured to receive a hydrocarbon fuel,e.g., natural gas, and serves as a source of the hydrocarbon fuel usedby reducing gas generator 14. Pressure regulator 40 is fluidly coupledto fuel inlet 38, and regulates the pressure of the hydrocarbon fuel.Sulfur capture sorbent 42 is fluidly coupled to pressure regulator 40,and is configured to capture sulfur from the fuel stream received frompressure regulator 40. Fuel flow controller 44 and fuel control valve 46are coupled to the output of sulfur capture sorbent 42, and areconfigured to control the amount of fuel delivered to merging chamber32.

Oxidant system 30 functions as an oxidant source for reducing gasgenerator 14, and includes an air intake 48, an air compressor 50 as apressurized air source, a pressure regulator 52, a nitrogen generator 54having a nitrogen separation membrane 56, a variable position/output aircontrol valve 58, an air flow controller 60, a variable position/outputoxidant control valve 62, an oxidant flow controller 64 and an oxygensensor 66.

Air intake 48 may be any structure or opening capable of providing air,and is fluidly coupled to air compressor 50, which compresses ambientair received from the atmosphere. Pressure regulator 52 is fluidlycoupled to air compressor 50, and regulates the air pressure deliveredto reducing gas generator 14. Air control valve 58 is part of an aircharging system structured to variably add air to the nitrogen-rich gasreceived from nitrogen generator 54 to yield an oxidant having avariable O₂ content.

The O₂ content may be sensed by oxygen sensor 66, which may be used bythe control system of reducing gas generator 14 to vary the O₂ contentof the oxidant supplied to merging chamber 32. For example, under normaloperating conditions, the O₂ content is controlled based on a controltemperature, e.g., the temperature of catalyst 36 in the presentembodiment, although other temperatures may be used in otherembodiments, e.g., the temperature of the reducing gas output byreducing gas generator 14. However, during startup of reducing gasgenerator 14, oxygen sensor 66 may be used to provide feedback until thetemperature is available as a feedback. The amount or flow of theoxidant having the variable O₂ content is controlled by oxidant controlvalve 62 and oxidant flow controller 64.

Nitrogen generator 54 is configured to generate a nitrogen-rich stream,which may be used as a purging gas, and which may also be combined withair to form a low oxygen (O₂) content oxidant stream, e.g., anitrogen-diluted air stream, used by reducing gas generator 14 to form areducing gas. The purity of the nitrogen-rich stream may vary with theneeds of the particular application, for example, and may consistessentially of nitrogen. Alternatively, it is considered that in otherembodiments, other gases may be employed in place of or in addition tonitrogen, such as argon or helium, for use as a purging gas and/or as aconstituent of a low O₂ content oxidant stream, e.g., as a dilutant(diluent) of air. As used herein, “low O₂ content oxidant” means thatthe oxygen content of the oxidant stream is less than that ofatmospheric air under the same pressure and temperature conditions.

Nitrogen generator 54 and air control valve 58 are fluidly coupled inparallel to pressure regulator 52, and receive pressurized air from aircompressor 50 for use in reducing gas generator 14 operations. Nitrogengenerator 54 has an output 54A, e.g., an opening or passage structuredto discharge the products of nitrogen generator 54. Nitrogen generator54 is structured to receive air from air intake 48, extract oxygen (O₂)from the air, and to discharge the balance in the form of anitrogen-rich gas from the outlet. The extracted O₂ is discharged fromnitrogen generator 54 to the atmosphere in the present embodiment,although it will be understood that in other embodiments, the extractedO₂ may be employed for other purposes related to fuel cell 12 and/orreducing gas generator 14, e.g., as part of an oxidant stream.

Nitrogen separation membrane 56 of nitrogen generator 54 is configuredto separate oxygen out of the air received from air intake 48, andprovides the nitrogen-rich stream, which is then combined with the airsupplied by air control valve 58 to yield the low O₂ content oxidant,which is delivered to oxidant control valve 62. Oxidant control valve 62is fluidly coupled to the outputs of both nitrogen generator 54 and aircontrol valve 58. Oxygen sensor 66, which may be in the form of an O₂analyzer, is fluidly coupled downstream to oxidant control valve 62, andprovides a control signal via control line 68 that communicativelycouples oxygen sensor 66 with air flow controller 60. Air flowcontroller 60 provides control signals to air control valve 58 tocontrol the amount of air added to the nitrogen-rich stream based on thecontrol input from oxygen sensor 66.

Merging chamber 32 is in fluid communication with the output of nitrogengenerator 54 and fuel input 38, and is structured to receive and combinethe hydrocarbon fuel and nitrogen-rich gas and discharge a feed mixturecontaining both the fuel and the oxidant including the nitrogen-rich gasto catalytic reactor 34. Catalytic reactor 34 is structured to receivethe feed mixture and to catalytically convert the feed mixture into areducing gas. The form of merging chamber 32 is a simple plumbingconnection joining the oxidant stream with the fuel stream in thepresent embodiment, although any arrangement that is structured tocombine an oxidant stream with a fuel stream may be employed withoutdeparting from the scope of the present invention. For example, adedicated mixing chamber having swirler vanes to mix the streams may beemployed.

Reducing gas generator 14 includes a start control valve 69 having avalve element 70 and a valve element 72; and a feed mixture heater 74,which may be used to start the process of generating reducing gas. Inone form, valve elements 70 and 72 are part of a combined valvingelement. The inlets of valve elements 70 and 72 are fluidly coupled tomerging chamber 32 downstream thereof. The outlet of valve element 70 isfluidly coupled to catalytic reactor 34 for providing the feed mixtureto catalyst 36 of catalytic reactor 34. The outlet of valve element 72is fluidly coupled to the inlet of feed mixture heater 74. In one form,start control valve 69 is a three-way valve that operates valve elements70 and 72 to direct flow entering valve 69 into catalytic reactor 34directly or via feed mixture heater 74. It is alternatively consideredthat other valve arrangements may be employed, such as a pair ofindividual start control valves in place of start control valve 69 withvalve elements 70 and 72.

Feed mixture heater 74 includes a heating body 76 and a flow coil 78disposed adjacent to heating body 76. The outlet of feed mixture heater74 is fluidly coupled to catalytic reactor 34 for providing heated feedmixture to catalyst 36 of catalytic reactor 34. In the normal operatingmode, valve elements 70 and 72 direct all of the feed mixture directlyto the catalytic reactor 34. In the startup mode, the feed mixture isdirected through feed mixture heater 74. In one form, all of the feedmixture is directed through feed mixture heater 74, although in otherembodiments, lesser amounts may be heated.

Feed mixture heater 74 is configured to “light” the catalyst 36 ofcatalytic reactor 34 (initiate the catalytic reaction of fuel andoxidant) by heating the feed mixture, which is then supplied tocatalytic reactor 34. In one form, the feed mixture is heated by feedmixture heater 74 to a preheat temperature above the catalyst light-offtemperature of the feed mixture (the catalyst light-off temperature isthe temperature at which reactions are initiated over the catalyst,e.g., catalyst 36). Once catalyst 36 is lit, the exothermic reactionstaking place at catalyst 36 maintain the temperature of catalyticreactor 34 at a controlled level, as set forth below. Also, oncecatalyst 36 is lit it may no longer be necessary to heat the feedmixture, in which case valve elements 70 and 72 are positioned to directall of the feed mixture directly to the catalytic reactor 34, bypassingfeed mixture heater 74.

In order to provide for a quick supply of reducing gas in the event of asudden shutdown of fuel cell 12, heating body 76 is configured tocontinuously maintain a temperature sufficient to light catalyst 36during normal power production operations of fuel cell 12. That is,while fuel cell 12 is operating in power production mode to supply powerto electrical load 16, which is the normal operating mode for fuel cell12, heating body 76 maintains a preheat temperature sufficient to heatthe feed mixture in order to be able to rapidly light the catalyst forstartup of reducing gas generator 14 so that reducing gas may besupplied to fuel cell 12 during shutdown.

In addition, one or more catalyst heaters 80 are disposed adjacent tocatalytic reactor 34, and are configured to heat catalyst 36 andmaintain catalyst 36 at a preheat temperature that is at or above thecatalyst light-off temperature for the feed mixture supplied tocatalytic reactor 34. This preheat temperature is maintained duringnormal operations of fuel cell 12 in power production mode in the eventof a sudden need for reducing gas, e.g., in the event of the need for ashutdown of fuel cell 12.

In other embodiments, it is alternatively considered that another heater82 may be used in place of or in addition to heaters 74 and 80, e.g., aheater 82 positioned adjacent to catalytic reactor 34 on the upstreamside. Such an arrangement may be employed to supply heat more directlyto catalyst 36 in order to initiate catalytic reaction of the feedmixture in an upstream portion of catalytic reactor 34.

In the present embodiment, heaters 74, 80 and 82 are electrical heaters,although it is alternatively considered that in other embodiments,indirect combustion heaters may be employed in addition to or in placeof electrical heaters. Also, although the present embodiment employsboth feed mixture heater 74 and heaters 80 to rapidly light the feedmixture on the catalyst, it is alternatively considered that in otherembodiments, only one such heater may be employed, or a greater numberof heaters may be employed, without departing from the scope of thepresent invention.

A control temperature sensor 84 is positioned adjacent catalyst 36 ofcatalytic reactor 34, and is structured to measure the temperature ofcatalyst 36. In one form, control temperature sensor 84 is structured toprovide a signal indicating the temperature of a portion of catalyst 36via a sense line 92 that communicatively couples air flow controller 60with control temperature sensor 84. The control temperature is atemperature employed by control system 96 in regulating the output ofreducing gas generator 14. Air flow controller 60 is configured todirect the operations of air control valve 58 based on the signalreceived from control temperature sensor 84 in conjunction with thesignal received from oxygen sensor 66. In another form, othertemperatures may be sensed for purposes of controlling reducing gasgenerator 14. For example, in one such embodiment, the temperature ofthe reducing gas produced by reducing gas generator 14, e.g., as outputby catalytic reactor 34, may be measured and used as a controltemperature feedback to direct the operations of air control valve 58.

A reducing gas combustibles detection sensor 86, which in the presentembodiment is in the form of a hydrogen (H₂) sensor or H₂ analyzer, isconfigured to determine the quantity of one or more combustibles, e.g.,percent mole, present in the reducing gas output by catalytic reactor34. In other embodiments, reducing gas combustibles detection sensor 86may be in the form of a carbon monoxide (CO) sensor or analyzer inaddition to or in place of the H₂ sensor/analyzer. In any case, acontrol line 94 communicatively couples fuel flow controller 44 andreducing gas combustibles detection sensor 86. Reducing gas combustiblesdetection sensor 86 is configured to supply a signal reflecting thecombustibles content of the reducing gas to fuel flow controller 44.Fuel flow controller 44 is configured to control the amount of fueldelivered to merging chamber 32.

The reducing gas output by catalytic reactor 34 is cooled using a heatexchanger 88. In one form, heat exchanger 88 is an indirect heatexchanger. In other embodiments, other types of heat exchangers may beemployed. In one form, reducing gas combustibles detection sensor 86 ispositioned downstream of heat exchanger 88. In other forms, reducing gascombustibles detection sensor 86 may positioned in other locations, forexample, upstream of heat exchanger 88 or inside of or mounted on heatexchanger 88.

The pressure output of catalytic reactor 34 is maintained by abackpressure regulator 90 downstream of heat exchanger 88. Heatexchanger 88 maintains the temperature of the reducing gas downstream ofcatalytic reactor 34 at a suitable level to prevent damage tobackpressure regulator 90. In one form, the reducing gas is cooled tobetween 100° C. and 150° C. using cooling air. In other embodiments,other suitable fluids may be used as the heat sink, and othertemperatures may be used. In one form, a control loop (not shown) may beused to control the temperature of the reducing gas exiting heatexchanger 88 by varying the flow of cooling air or other cooling fluid.

The output of reducing gas generator 14 is fluidly coupled to catalyticreactor 34, and is in fluid communication with anode 20, e.g., eitherdirectly or via reformer 26. The output of backpressure regulator 90serves as a reducing gas output in the present embodiment, and isoperative to direct the reducing gas to anode 20 and reformer 26. The“reducing gas output” is the output of reducing gas generator 14 thatdischarges the product of reducing gas generator 14 into fuel cell 12,and may be one or more of any opening or passage structured to dischargethe products of reducing gas generator 14.

Fuel flow controller 44, air flow controller 60 and oxidant flowcontroller 64 form a control system 96 that is structured to control thetemperature and chemical makeup of the product mixture supplied fromcatalytic reactor 34 based on the signals output by oxygen sensor 66(during startup in the present embodiment), control temperature sensor84 and reducing gas combustibles detection sensor 86. In particular, aircontrol valve 58 is controlled by air flow controller 60 to regulate theO₂ content of the oxidant stream supplied to merging chamber 32, e.g.,the amount of O₂ expressed as a mole percentage of the O₂ in the oxidantstream. Oxidant control valve 62 is controlled by oxidant flowcontroller 64 to regulate flow of the oxidant stream formed ofnitrogen-rich gas and air supplied to merging chamber 32. Fuel controlvalve 46 is controlled by fuel flow controller 44 to regulate the amountof hydrocarbon fuel supplied to merging chamber 32.

Thus, in the present embodiment, control system 96 is configured tocontrol the oxygen (O₂) content of the oxidant stream, and to alsocontrol the oxidant/fuel ratio of the feed mixture, which is defined bya ratio of the amount of the oxidant in the feed mixture to the amountof hydrocarbon fuel in the feed mixture, e.g., the mass flow rate of theoxidant stream relative to the mass flow rate of the hydrocarbon fuelstream. In particular, the O₂ content of the oxidant stream supplied tomerging chamber 32 is controlled by air control valve 58 via the outputof air flow controller 60 based on the signal received from oxygensensor 66. In addition, the oxidant/fuel ratio of the feed mixturesupplied to catalytic reactor 34 is controlled by fuel control valve 46and oxidant control valve 62 under the direction of fuel flow controller44 and oxidant flow controller 64, respectively. In one form, the flowof reducing gas output by reducing gas generator 14 is controlled byoxidant control valve 62, e.g., including an offset or othercompensation to account for the amount of fuel in the feed mixture,whereas the oxidant/fuel ratio is then controlled using fuel controlvalve 46. In other embodiments, other control schemes may be employed.

In the present embodiment, each of fuel flow controller 44, air flowcontroller 60 and oxidant flow controller 64 are microprocessor-based,and execute program instructions in the form of software in order toperform the acts described herein. However, it is alternativelycontemplated that each such controller and the corresponding programinstructions may be in the form of any combination of software, firmwareand hardware, and may reflect the output of discreet devices and/orintegrated circuits, which may be co-located at a particular location ordistributed across more than one location, including any digital and/oranalog devices configured to achieve the same or similar results as aprocessor-based controller executing software or firmware basedinstructions, without departing from the scope of the present invention.Further, it will be understood that each of fuel flow controller 44, airflow controller 60 and oxidant flow controller 64 may be part of asingle integrated control system, e.g., a microcomputer, withoutdeparting from the scope of the present invention.

In any event, control system 96 is configured to execute programinstructions to both vary the O₂ content of the oxidant stream and varythe oxidant/fuel ratio of the feed mixture while maintaining a selectedtemperature of the reducing gas in order to achieve a selectedcombustibles content at desired flow rate. The flow rate may be varied,e.g., depending upon the particular application or operational phase.Control system 96 varies the O₂ content of the oxidant stream and theoxidant/fuel ratio of the feed mixture based on the output of controltemperature sensor 84, oxygen sensor 66 and reducing gas combustiblesdetection sensor 86.

Reducing gas generator 14 may be employed during startup and shutdown offuel cell 12, e.g., to provide reducing gas of various reducingstrengths, including reducing gas in the form of a safe (non-flammable)gas, and in some embodiments, to provide a purging gas with nocombustibles.

The reducing gas is generated by combining the nitrogen-rich stream withair supplied via air control valve 58 to form the oxidant stream, whichis regulated by oxidant control valve 62 and combined with thehydrocarbon fuel supplied via fuel control valve 46 to form the feedmixture that is catalytically converted in catalytic reactor 34 into thereducing gas. As set forth herein, the O₂ content of the oxidant streamand the oxidant fuel ratio of the feed mixture are varied by controlsystem 96 in order to both regulate the control temperature, e.g., atcatalytic reactor 34, while also controlling the reducing strength ofthe reducing gas to achieve the selected combustibles content at thedesired flow rate.

The combustibles content may be selected in order to provide theappropriate reducing gas chemical configuration during various phases inthe fuel cell 12 startup and shut down processes. In the presentembodiment, control system 96 is structured to maintain the controltemperature, e.g., the catalyst 36 temperature, while varying thecombustibles content. For example, the reducing strength may be variedfrom weakly reducing, i.e., a low reducing strength, for purposes offorming a safe gas, to a high reducing strength having greatercombustibles content. The combustibles content is primarily in the formof hydrogen (H₂) and carbon monoxide (CO).

The safe gas may be supplied to fuel cell 12 during ramp up to fuel cell12 operating temperature. In one form, the reducing gas may be suppliedto fuel cell 12 in the form of a safe gas to transition reformer 26 intoservice. In another form, as the operating temperature of fuel cell 12increases, e.g., the temperature of anode 20 and reformer 26, thestrength of the reducing gas may be increased by increasing thecombustibles content of the reducing gas, which may thus protect anode20 at the higher temperatures at which a significant amount of oxidationdamage may otherwise occur, e.g., due to oxygen migration throughelectrolyte 22 or other leakages. In addition, as anode 20 (and/orreformer 26, in some embodiments) approaches normal operatingtemperatures, the combustibles content of the reducing gas may befurther increased to achieve combustibles content levels similar to thatof the synthesis gas that is produced by reformer 26 during normal powergeneration operations of fuel cell 12, which may help initiate thenormal electrical power-producing reactions of anode 20. In embodimentswhere supplied to reformer 26, this may help initiate the normaloperating catalytic reactions of reformer 26.

Regarding the purging gas, in some embodiments, a noncombustible purginggas may be generated by nitrogen generator 54 in the form of anitrogen-rich stream, e.g., consisting primarily of nitrogen, which maysupplied to fuel cell 12 via back pressure regulator 90, although otherplumbing schemes to direct the output of nitrogen generator 54 to fuelcell 12 may alternatively be employed. In one form, the purging gas maybe supplied to fuel cell 12, e.g., to purge one or more of cathode 24and/or other fuel cell 12 components, e.g., when a cold start of fuelcell 12 is desired. In another form, the purging gas may be supplied tofuel cell 12 to purge fuel cell 12 before maintenance. In yet anotherform, nitrogen generator 54 and/or a second nitrogen generator may beemployed to create a purge gas. For example, in the event of a loss ofthe power plant's main air supply during an emergency shut-down, anitrogen rich cathode purge may be supplied to cathode 24 with, e.g.,using nitrogen generator 54 and/or a second nitrogen generator, whilenitrogen generator 54 is used to generate the reducing gas supplied tothe anode 20 loop. Such embodiments may be used to ensure that “safe”non-flammable mixtures reside in the fuel cell 12 vessel.

Having thus described exemplary means for varying the combustiblescontent of the reducing gas output by catalytic reactor 34 whilemaintaining a constant reducing gas output temperature from catalyticreactor 34, including means for varying the O₂ content in oxidantsupplied to merging chamber 32 and means for varying the oxidant/fuelratio of feed mixture exiting merging chamber 32, an exemplaryembodiment of a method for generating a purging gas and a reducing gasfor startup and shutdown of a fuel cell is described as follows. Theexemplary embodiment is described with respect to FIGS. 3A-3D, whichform a flowchart having control blocks B100-B146 depicting a method forstarting up and shutting down a fuel cell. Although a particularsequence of events is illustrated and described herein, it will beunderstood that the present invention is not so limited, and that othersequences having the same or different acts in lesser or greater numbersand in the same or different order may be employed without departingfrom the scope of the present invention.

Referring now to FIG. 3A, at block B100, a command to start fuel cell 12is received by control system 96, e.g., via an operator of fuel cell 12.

At block B102, a bypass system 98 is engaged. Bypass system 98 opens avent line to vent the output of reducing gas generator 14, and closesthe flowpath to fuel cell 12. The output of reducing gas generator isvented until the control loop, e.g., control system 96, holds processparameters within their prescribed bounds, at which point bypass system98 closes the vent line and opens the flowpath to fuel cell 12.

At block B104, air is supplied to reducing gas generator 14, e.g., viaair intake 48, by initiating operation of air compressor 50.

At block B106, air compressor 50 compresses the air received from airintake 48. In one form, the air is compressed to a pressure in a rangefrom 5 bar absolute to 14 bar absolute. In other embodiments, thepressure of the compressed air may fall within a different range, forexample, in a range from 2 bar absolute to 25 bar absolute in someembodiments, and in other embodiments, 1 bar absolute to 30 barabsolute. The pressure supplied by air compressor 50 may vary, forexample, depending upon the characteristics of nitrogen separationmembrane 56 and nitrogen generator 54.

At block B108, the nitrogen-rich gas stream is generated in nitrogengenerator 54 of reducing gas generator 14 by supplying the compressedair to nitrogen separation membrane 56. The O₂ removed from the air bynitrogen separation membrane 56 as a byproduct of the nitrogengeneration process is directed offboard, e.g., for use elsewhere, orsimply vented, whereas the resulting nitrogen-rich stream is directedtoward oxidant control valve 62. In the present embodiment, thenitrogen-rich stream contains oxygen, albeit at levels lower than thatof ambient air. In other embodiments, the nitrogen stream may consistessentially of nitrogen (e.g., <1% O₂).

At block B110, compressed air is added to the nitrogen-rich stream in acontrolled manner by air control valve 58 under the direction of airflow controller 60 to form a low oxygen (O₂) content oxidant stream,i.e., an oxidant stream having less O₂ than ambient atmospheric air.

At block B112, a flow of hydrocarbon fuel to reducing gas generator 14is initiated by fuel control valve 46 under the direction of fuel flowcontroller 44. Fuel flow may be initially set to a default valueanticipated to achieve the desired combustibles content of the reducinggas and the control temperature, and may be subsequently adjusted.

At block B114, the oxidant stream is combined with the hydrocarbon fuelstream in merging chamber 32 to form the feed mixture having anoxidant/fuel ratio, e.g., defined by a ratio of the mass flow rate ofthe oxidant stream in the feed mixture to the mass flow rate of thehydrocarbon fuel stream in the feed mixture.

Referring now to FIG. 3B, at block B116, heating devices are operated ata temperature at or above the catalyst light-off temperature of the feedmixture, and the heat output by the heating devices is supplied to thefeed mixture. In one form, the heating devices are turned on immediatelyafter receiving the command to start the fuel cell 12, e.g., immediatelyafter block B100. In other embodiments, the heating devices may beturned on at other times suitable to the application, e.g., dependingupon how much time it takes the heaters to reach the desiredtemperature. In the present embodiment, the heating devices are feedmixture heater 74 and heater 80, although in other embodiments, only oneheater may be employed or a plurality of heaters may be employed inplace of or in addition to one or both of feed mixture heater 74 andheater 80. The types or forms of heaters used in other embodiments mayvary with the needs of the application.

Heating body 76 and flow coil 78 are maintained at or above the catalystlight-off temperature of the feed mixture. The heat from heating body 76and flow coil 78 is supplied to the feed mixture by diverting feedmixture through feed mixture heater 74, in particular, flow coil 78. Inone form, all of the feed mixture is diverted through feed mixtureheater 74. In another form, a portion of the feed mixture is divertedthrough feed mixture heater 74. The feed mixture is diverted to flowcoil 78 by controlling the output of start control valve 69 to operatevalve elements 70 and 72. The resulting heated feed mixture is directedto catalyst 36 of catalytic reactor 34 to help initiate the catalyticreactions that yield reducing gas. Once the catalytic reactions incatalytic reactor 34 have been started, three-way start control valve 69is re-oriented to direct all of the feed mixture directly to catalyticreactor 34, bypassing feed mixture heater 74. While the presentapplication is described using a feed mixture heater 74 with heatingbody 76 and flow coil 78, it will be understood that other types ofheaters may be employed in embodiments that utilize a flow mixtureheater.

Heater 80 of the present embodiment is in the form an electric bandheater, and maintains catalyst 36 at or above the catalyst light-offtemperature of the feed mixture, thereby promoting rapid lighting(hence, re-lighting) of catalyst 36. It will be understood that othertypes of heaters may be employed without departing from the scope of thepresent invention.

In other embodiments, heater 82 may be employed to heat catalyst 36 ator near the location where the feed mixture is supplied to catalyst 36in order to initiate the catalytic reactions. In various otherembodiments, one or more heaters 82 may be used in place of or inaddition to heaters 74 and 80.

At block B118, the heated feed mixture is directed to catalyst 36, wherecatalytic reactions are initiated. In one form, the catalytic reactionsare initiated based on the heat received from feed mixture heater 74. Invarious other forms, the reactions may be initiated based on heatreceived from feed mixture heater 74 and/or heater 80 and/or heater 82).

At block B120, the feed mixture is catalytically converted to reducinggas in catalytic reactor 34 of reducing gas generator 14.

At block B122, the O₂ content of the oxidant stream and the oxidant/fuelratio of the feed mixture are each controlled by control system 96 tomaintain the selected control temperature of the reducing gas and toyield the reducing gas in the form of a safe gas. In one form, the O₂content of the oxidant stream is controlled by air flow controller 60directing the operations of air control valve 58, although in otherembodiments, the O₂ content of the oxidant stream may be controlleddifferently. In one form, the oxidant/fuel ratio is controlled by fuelflow controller 44 directing the operations of respective fuel controlvalve 46, although in other embodiments, the oxidant/fuel ratio may becontrolled differently. Prior to reaching the control temperature,control of the O₂ content may be based on the output of oxygen sensor66. Once a temperature indicating catalytic combustion is achieved, thecontrol algorithm switches to feedback based on control temperaturesensor 84. The control temperature in some embodiments may be, forexample, a function of reducing gas flow rate (catalyst load), time atservice, or some other operating parameter. In other embodiments, theoutput of either or both of oxygen sensor 66 and control temperaturesensor 84 may be employed during system startup and/or normal operation.

The flow rate of the feed mixture is controlled primarily by oxidantflow controller 64 directing the operations of oxidant control valve 62.In the form of a safe gas, i.e., a weakly reducing gas mixture, thereducing gas may have a combustibles content (e.g., predominantly CO+H₂)of approximately 4.5%. Other reducing gases having greater or lesserpercentages of combustibles content may be employed without departingfrom the scope of the present invention.

Because the mass flow of the feed mixture is based predominantly on theflow rate of the oxidant flow stream, the total flow of the feedmixture, and hence the reducing gas output by reducing gas generator 14,is based primarily on the flow rate of the oxidant control flow streamas governed by oxidant flow controller 64. The selected controltemperature in the present embodiment is 800° C., which is measured atone of the hottest points in catalyst 36, and which in the presentembodiment yields a bulk average temperature of 770° C. The selectedtemperature in the present embodiment is a predetermined temperaturevalue selected based on life considerations for components of reducinggas generator 14 and fuel cell 12, as well as catalytic conversionefficiency. Other temperature values and measurement locations may beemployed in other embodiments.

At block B124, bypass system 98 is disengaged from the bypass mode, andthe reducing gas in the form of a safe gas is thus directed fromreducing gas generator 14 to anode 20 of fuel cell 12. In otherembodiments, the safe gas may be directed to reformer 26.

Referring now to FIG. 3C, a block B126 is illustrated. In one form,block B126 is bypassed, and process flow proceeds directly to blockB128. In another form, at block B126 the O₂ content of the oxidantstream and the oxidant/fuel ratio of the feed mixture are controlled toselectively vary the reducing strength of the reducing gas byselectively varying the combustibles content of the reducing gas whilemaintaining the selected temperature of the reducing gas of block B122.As set forth above with respect to block B122, in one form, the O₂content of the oxidant stream is controlled by air flow controller 60directing the operations of air control valve 58. In other forms, the O₂content of the oxidant stream may be controlled differently. In oneform, the oxidant/fuel ratio is primarily controlled by fuel flowcontroller 44, and the reducing gas flow is primarily controlled byoxidant flow controller 64 directing the operations of oxidant controlvalve 62. In other forms, the oxidant/fuel ratio and reducing gas flowrate may be controlled differently.

Control of the O₂ content of the oxidant stream and of the oxidant/fuelratio of the feed mixture to selectively vary the reducing strength ofthe reducing gas while maintaining the selected temperature and flowrate of the reducing gas output by catalytic reactor 34 in the presentembodiment is now described.

Reducing gas generator 14 catalytically converts the low O₂ contentoxidant and hydrocarbon fuel to form the reducing gas with sufficientcombustibles content to protect fuel cell anode 20 of fuel cell 12during start-up and shutdown of the fuel cell system 10 power plant. Byadjusting the O₂ content of the oxidant gas in combination with changingthe oxidant/fuel ratio, the reducing gas strength may be changed whilethe catalyst operating temperature is held constant, e.g., at an idealconversion temperature. This temperature is sensed by controltemperature sensor 84 and used as input to control system 96 for use inmaintaining the output temperature of catalytic reactor 34 at theselected temperature.

Referring now to FIG. 4, an example of catalytic reactor 34 parametersis depicted. The illustrated parameters include oxidant stream mass flowrate 100; hydrocarbon fuel stream mass flow rate 102; percent (%)stoichiometric air 104, which represents the percentage amount of air inthe oxidant stream relative to the amount of air required for completecombustion of the hydrocarbon fuel stream; and the oxygen/carbon ratio(O₂/C) 106. In the plot of FIG. 4, the abscissa is H₂ content of thereducing gas, the left-hand ordinate is in units of percent and alsograms per second (g/s), against which % stoichiometric air 104 andoxidant stream mass flow rate 100 are plotted. The right-hand ordinateis in units of both molar fraction and g/s, against which O₂/C ratio 106and hydrocarbon fuel stream mass flow rate 102 are plotted.

FIG. 4 illustrates catalytic reactor 34 operating parameters over areducing gas compositional range of 2% to 20% H₂ and 1% to 10% CO (3% to30% CO+H2). To produce higher combustibles content (CO+H₂), the O₂content in the oxidant is raised. At a constant oxidant/fuel ratio ofthe feed mixture, e.g., air to fuel ratio, raising the O₂ content in theoxidant stream reduces combustibles and raises operating temperature.However, in the present embodiment, as the O₂ content in the oxidantstream is increased, the oxidant/fuel ratio of the feed mixture issimultaneously decreased, i.e., made more fuel rich, in order to achievehigher combustibles content at the same operating temperature.

By varying both the O₂ content in the oxidant stream and theoxidant/fuel ratio of the feed mixture, a broad range of reducing gasstrengths may be achieved at a selected catalyst operating temperature,e.g., 770° C. in the present embodiment. For example, in one form, therange may extend from a reducing gas strength that represents normaloperating conditions for reformer 26 (−45% CO+H₂) to weakly reducingconditions (˜3% CO+H₂). In other forms, different ranges may beemployed, e.g., as set forth herein.

As 20% H₂ content in the reducing gas is approached, conditions incatalytic reactor 34 may approach that normally occurring in reformer 26in power production mode as the oxidant approaches air with respect to %O₂ content and the O₂ to C molar ratio reaches 0.65. As the reducing gasbecomes richer in combustibles, the fuel flow may increase by a factorof about 4 at 20% H₂ relative to weakly reducing conditions. Thepercentage of the fuel burned may decrease significantly as conditionsapproach those in the reformer 26. The temperature may be sustainedbecause the lower percentage of combustion oxygen is offset by thecombination of the elevated fuel flow rate and the decreased heatdissipation through less N₂ dilution in the oxidant. Thus, even thoughthe O₂ concentration in the oxidant increases for increased reducingstrength, as a percentage of oxygen required to completely consume thefuel, the oxygen level decreases. In the present embodiment, percent COcontent is about ½ of the percent of H₂ content at the desired operatingtemperature, and hence the combustibles content of the reducing gas isapproximately 1.5 times the percent of H₂ content in the reducing gas.While described in the present application with respect to a fuel cellsystem, it will be understood that reducing gas generator 14 is equallyapplicable to other systems, such as systems for generating reducing gasfor other purposes.

Referring again to FIG. 3C, at block B128, the reducing gas is suppliedto reformer 26, and to anode 20, e.g., via reformer 26.

At block B130, a transition of fuel cell 12 into power production modeis initiated, which includes supplying to fuel cell 12 flows of theprimary fuel and the primary oxidant that are normally provided to fuelcell 12 for operation in power production mode, in contrast to theoxidant and hydrocarbon fuel provided to reducing gas generator 14 togenerate reducing gas for use during startup or shutdown of fuel cell12. The transition into power production mode also includes heatingportions of fuel cell 12, including anode 20 and reformer 26, to normaloperating temperature in a controlled fashion so as to reduce mechanicalstresses that might result from thermal gradients within and betweensuch components. The heating of fuel cell 12 may be performed prior to,during and after the provision of reducing gas to fuel cell 12, and maybe performed until satisfactory operating temperatures in such portions,e.g., anode 20 and reformer 26, are achieved. During the transition intopower production mode, bypass system 98 may be transitioned into bypassmode.

At block B132, fuel cell 12 is operated in power production mode, i.e.,normal operating mode, to supply power to electrical load 16.

At block B134, the airflow and fuel flow supplied to reducing gasgenerator 14 are terminated, ending the production of reducing gas byreducing gas generator 14.

Referring now to FIG. 3D, at block B136, the temperature of the heatingdevice is maintained at or above the temperature required to initiatecatalytic reaction of the feed mixture at catalyst 36. This temperatureis maintained during operation of the fuel cell in the power productionmode, e.g., in order to provide for rapid restart of reducing gasgenerator 14, including rapid restart of catalyst 36, in the event of aneed to shut down fuel cell 12.

At block B138, a command to shut down fuel cell 12 from the powerproduction mode is received by control system 96, e.g., via a humaninput or an automated process. It will be noted that in someembodiments, block B136 may be performed subsequent to receiving thecommand to shut down fuel cell 12. For example, in some embodiments, theheating device may be not be heated to a temperature at or above thecatalytic light-off temperature until the command to shutdown fuel cell12 is received.

At block B140, reducing gas generator 14 generates reducing gas inresponse to the command, e.g., by performing some or all of the actionsindicated above with respect to blocks B102 to B128, includingcontrolling the O₂ content of the oxidant stream and the oxidant/fuelratio of the feed mixture to selectively vary the reducing strength ofthe reducing gas by selectively varying the combustibles content of thereducing gas to a desired level while maintaining a selectedtemperature, e.g., the selected temperature of block B122, above.

At block B142, the reducing gas generated by reducing gas generator 14is supplied to anode 20 of fuel cell 12 by disengaging bypass system 98from the bypass mode. This may help to prevent oxidation damage to anode20 during shutdown of fuel cell 12. Initially, the reducing gas may havea high reducing strength, which may be decreased as the temperature offuel cell 12 decreases.

At block B144, a transition of fuel cell 12 out of the power productionmode is initiated, including gradually reducing the flow to anode 20 ofthe primary fuel that is normally provided during operation in powerproduction mode.

At block B146, the airflow and fuel flow supplied to reducing gasgenerator 14 are terminated, ending the production of reducing gas byreducing gas generator 14. Block B146 may be executed after anode 20 issufficiently cooled to a temperature at which oxidative damage is not aconcern, which may vary with the materials used to manufacture anode 20.

A reducing gas generator in accordance with some embodiments of thepresent application may include a compressed air supply that feeds apolymer nitrogen-separation membrane, which uses the high pressure tosegregate oxygen from nitrogen across a polymer fiber. Such embodimentsmay preclude the need for bottled nitrogen. In other embodiments, othernitrogen sources may be employed. The product gas is a nitrogen-richstream that is depleted in oxygen. A variable-position bypass valve maydivert a relatively small stream of the feed air around the nitrogengenerator for blending with the nitrogen-rich stream. In someembodiments, the bypass airflow is directly proportional to the finaloxygen content of the blended streams. The blended stream ofnitrogen-rich product gas and bypass air may be referred to as anoxidant stream, which passes through a flow control device that sets theflow of oxidant to the process. The bypass valve controls theproportions of bypass air and nitrogen-rich gas to achieve the desiredoxygen content of the oxidant stream.

A relatively small quantity of hydrocarbon fuel may be metered into theoxidant stream through a flow control device. In a steady state flowmode, the premixed oxidant and fuel blend is fed directly into acatalytic reactor that converts the feed mixture into the reducing gas.Compared with ordinary combustion in air, the reduced oxygen contentoxidant stream may translate to less fuel per unit combustibles yield inthe reducing gas. Thus, the required chemical energy input (i.e., thethermal load due to the input of fuel) per unit production ofcombustibles (e.g., H₂ and CO) may also be decreased, and therefore,less heat may need to be extracted from the process gas to cool theproduct stream to a required temperature. The nitrogen dilution of theoxidant stream may also decrease the reaction temperature into the rangethat may be preferable for the catalyst, and may not exceed the materiallimits in the downstream heat exchanger. In contrast to embodiments ofthe present invention, a reactor designed for combustion with normal air(in contrast to the nitrogen-rich oxidant employed in embodiments of thepresent invention) at the required scale might be complex, and mightrequire cooling jackets that would likely require a liquid coolant, orotherwise a very high volumetric flow of coolant gas, and therefore,would have a relatively large heat duty in order to protect reactormaterials from excessive temperature. In contrast, the catalytic reactorof some embodiments of the present invention may be designed to operateat a lower temperature without the need for external cooling.

Fuel oxidation with an oxygen-depleted oxidant may yield a given rangeof combustibles concentration (or molar flow) over a much wider range ofair to fuel ratio relative to ordinary combustion with air, which makescontrol of the combustibles content easier to achieve.

Thermocouple(s) may monitor the exit temperature at the catalyst exit.The thermocouple may act as the control input for the air bypass valve.If the exit temperature were to fall too far below the set point, acontrol signal would open the bypass by some amount since an oxidantstream having a higher proportion of O₂ elevates the exit temperature(by oxidizing more fuel) and vice versa. The set point temperature isset high enough to achieve complete conversion of the flammable feedmixture to the equilibrated gas composition, but not too high as toapproach the operational material limit temperatures for either thecatalyst or the downstream heat exchanger.

An oxygen sensor 66 may measure the oxygen content on a volume basis ofthe oxidant stream downstream of the mix point for the bypass air andthe nitrogen-rich stream exiting the nitrogen generator. An alternativeembodiment may employ the measured oxygen concentration rather than theexit temperature to position air bypass control valve so that the exittemperature is maintained to a set point value. This may be preferableat start-up before a representative steady state reactor exittemperature is available to set the bypass valve position.

The oxygen sensor may be a small zirconia sensor maintained at a hightemperature, e.g., around 600° C. for some embodiments, which develops aNernst potential when exposed to oxygen, which is related to the oxygencontent of the gas. The sensor can be located in-situ. However, thesensor may alternatively be submerged in a controlled small slip streamthat is blown down off the main process line through a critical floworifice. The control software may dictate the relationship between thedeviation of the measured oxygen content from the targeted value, andthe incremental amount the bypass valve is opened as a result. Thesensor may have a rapid response to changes in the oxygen content of theprocess gas, and therefore, the optimized tuning parameters on the airbypass valve control loop may provide more reliable control over abroader range of conditions.

The downstream heat exchanger cools the reducing gas to a temperaturethat is required for introduction of the reducing gas into thedownstream process. A temperature control loop may vary a flow ofcooling air or other cooling medium to the heat exchanger based on thedeviation of the catalyst exit temperature from the temperature setpoint of the outlet gas. The heat exchanger may be a compact alloy steelor ceramic design to withstand the temperature of the gas exiting thecatalyst.

A hydrogen or combustibles sensor may extract a slipstream of theprocess gas downstream of the heat exchanger to measure the percent byvolume hydrogen or combustibles as a constituent of the reducing gas.The control software may compare the measured % H₂ to a set point value,and based on the difference sends a control signal to fuel controlvalve. If the measured % H₂ deviates too far below the set point, thefuel feed would be increased, and vice versa. The control software maydictate the relationship between the deviation of the measured % H₂ withthe targeted % H₂, and the incremental amount the fuel valve is openedor closed.

One approach for continuously measuring hydrogen uses a thermalconductivity hydrogen sensor calibrated over the permissible range ofhydrogen content for the reducing gas. Similar to the oxygen sensor, acritical flow orifice may be used as a relatively inexpensive and simpleway to meter a very small slipstream of the reducing gas at the correctsample gas flow to the sensor.

A method for rapid restart of the catalyst from a standby condition tobring the reducing gas generator back on-line as quickly as possible forunforeseen events within the fuel cell system that will require animmediate supply of safe reducing gas may also be provided byembodiments of the present invention. A rapid restart capability mayavoid the need for a bottled storage of reducing-gas necessary to bridgethe gap between the time that the gas is demanded and the time requiredto bring the reducing gas generator on-line. A rapid restart method mayemploy a heater with a high thermal mass located just upstream of thecatalyst reactor and, e.g., a pair of valves or a three-way valve fordiverting feed mixture flow through the heater. During normal operationthe valve directs the mixture directly into the catalytic reactor,bypassing the heater. At start-up, flow may be diverted through theheater. In the absence of flow, e.g., under idle conditions of thereducing gas generator, the heater is continuously supplied sufficientpower to sustain the metal at the desired preheat temperature whilebalancing a relatively small heat loss, and thus, this power demand maybe small. Within the heater, a flow coil may be engulfed with a metallicbody. The heater may contain sufficient thermal mass so that when flowis initiated upon a re-start attempt, the process stream immediatelyacquires the targeted ignition temperature.

Such a design may be relatively safe because it may achieve goodelectrical isolation between the flammable mixture and the power supplythat acts on the metallic body. Prior to a re-start sequence, the heaterregulates power to the internal metal to the required temperature priorto the introduction of flow, and must only maintain power to offset heatloss through the surrounding insulation at this condition.

On a start-up attempt, power may be immediately ramped up to sustain orelevate the set-point preheat temperature until reaction of the catalystfeed mixture is achieved. Once this is achieved, e.g., as indicated by asufficient rise in temperature at the catalyst exit, the flow may bediverted around the ignition heater directly into the catalyst (normaloperating flow mode) to prevent overheating of the catalyst.

To further promote rapid re-start, band heaters may provide anadditional heat source. The band heaters may surround the catalystreactor to hold the catalyst at or above the catalyst light-offtemperature before flow is initiated at start-up. Prior to start-up, theband heaters would preferably provide the energy to offset heat lossthrough the insulation surrounding the band heaters. Once the catalystis lit, the band heaters may turn off as the skin temperature risesabove the set point temperature of the heaters. Power to the heater maybe either turned off or turned down to sustain the heater's thermal massat the temperature set point for the next restart.

Other alternative embodiments would simplify the heat-up scheme byemploying a closely coupled heater at the catalyst inlet. This approachmay use a low thermal mass heater that would locally initiate reactionnear the front side of the catalyst by close thermal coupling, which insuch embodiments may potentially reduce the reducing gas generator'spart count and cost.

In an additional embodiment, the reducing gas generator may replace theinternal reformer for the fuel cell system for those embodiments wherethe reducing gas generator is structured to produce a reducing gas thatis suitable for power production in the fuel cell system. In some suchembodiments, the reduced gas generator may be used for producing areducing gas of one composition for startup and shutdown of the fuelcell system, and for producing a reducing gas of an alternatecomposition for the normal operation of the fuel cell system.

Embodiments of the present invention may include a method for startupand shutdown of a fuel cell using a reducing gas generator. The methodmay include receiving air into a reducing gas generator air intake;generating a low oxygen (O₂) content oxidant stream using at least inpart air received from the air intake; and initiating flow of ahydrocarbon fuel stream in the reducing gas generator. The method mayalso include combining the hydrocarbon fuel stream with the low O₂content oxidant stream to yield a feed mixture; directing the feedmixture to a catalyst; and catalytically converting the feed mixtureinto a reducing gas. The method may also include directing the reducinggas to at least one of an anode and a reformer of the fuel cell, andinitiating a transition of the fuel cell into a power production mode,or a transition of the fuel cell out of the power production mode.

One refinement of the embodiment may include the transition of the fuelcell into the power production mode which may include supplying flows ofa primary fuel and a primary oxidant to the fuel cell. The transition ofthe fuel cell out of the power production mode may include terminatingthe flows of the primary fuel and the primary oxidant to the fuel cell.

Another refinement of the embodiment may include directing the reducinggas to the anode of the fuel cell which may include directing thereducing gas to the anode via the reformer.

Another refinement of the embodiment may include generating anitrogen-rich stream from the air received from the air intake. Thenitrogen-rich stream may form at least a part of the low oxygen (O₂)content oxidant stream.

Another refinement of the embodiment may include maintaining a selectedcontrol temperature by independently varying both the O₂ content of theoxidant stream and the oxidant/fuel ratio of the feed mixture.

Another refinement of the embodiment may include sensing the controltemperature.

Another refinement of the embodiment may include controlling theselected control temperature based on the sensed control temperature.

Another refinement of the embodiment may include sensing the O₂ contentin at least one of the oxidant stream and the feed mixture.

Another refinement of the embodiment may include controlling theselected control temperature based on the sensed O₂ content.

Another refinement of the embodiment may include sensing a combustiblescontent in the reducing gas.

Another refinement of the embodiment may include controlling theselected control temperature based on the sensed combustibles content.

Another refinement of the embodiment may include controlling a flow rateof the feed mixture while performing the maintaining of the selectedcontrol temperature by varying both the O₂ content of the oxidant streamand the oxidant/fuel ratio of the feed mixture.

Another refinement of the embodiment may include selectively varying acombustibles content of the reducing gas while maintaining a selectedcontrol temperature by independently varying both the O₂ content of theoxidant stream and the oxidant/fuel ratio of the feed mixture.

Another refinement of the embodiment may include sensing a controltemperature.

Another refinement of the embodiment may include selectively varying thecombustibles content of the reducing gas while maintaining the selectedcontrol temperature is performed based on the sensed controltemperature.

Another refinement of the embodiment may include the sensing the O₂content in at least one of the oxidant and the feed mixture.

Another refinement of the embodiment may include selectively varying thecombustibles content of the reducing gas while maintaining the selectedcontrol temperature based on the sensed O₂ content.

Another refinement of the embodiment may include sensing thecombustibles content in the reducing gas.

Another refinement of the embodiment may include selectively varying thecombustibles content of the reducing gas while maintaining the selectedcontrol temperature based on the sensed combustibles content.

Another refinement of the embodiment may include controlling a flow rateof the feed mixture while performing the selectively varying thecombustibles content of the reducing gas while maintaining the selectedcontrol temperature.

Another refinement of the embodiment may include the reducing gasinitially produced as a safe gas directed to the fuel cell during thetransition of the fuel cell into the power production mode, furthercomprising increasing the reducing strength of the reducing gas byincreasing the combustibles content of the reducing gas.

Another refinement of the embodiment may include heating at least aportion of the feed mixture; directing the heated feed mixture to thecatalyst; and heating the catalyst using the heated feed mixture.

Another refinement of the embodiment may include maintaining a surfaceat a preheat temperature one of at and above a catalyst light-offtemperature of the feed mixture; and directing the at least a portion ofthe feed mixture across the surface in order to perform the heating.

Another refinement of the embodiment may include preparing for ashutdown of the fuel cell by maintaining the surface at the preheattemperature while the fuel cell is operating in the power productionmode.

Another refinement of the embodiment may include heating the catalyst toa preheat temperature configured for catalytic auto-ignition of the feedmixture (e.g., catalytic light-off temperature) prior to directing thefeed mixture to the catalyst.

Another refinement of the embodiment may include heating at least aportion of the feed mixture prior to directing the feed mixture to thecatalyst.

Another refinement of the embodiment may include generating anitrogen-rich stream from the air received from the air intake; anddirecting at least some of the nitrogen-rich stream to purge a componentassociated with a fuel cell.

Another refinement of the embodiment may be the oxidant stream whichincludes an inert gas selected from the group consisting of nitrogen,argon and helium.

Another embodiment of the present invention may include a method forgenerating a reducing gas having a variable combustibles content. Themethod for generating a reducing gas having a variable combustiblescontent may include providing a nitrogen-rich oxidant stream. Theoxidant stream may have a nitrogen content greater than that of ambientair. The oxidant stream may also have an oxygen (O₂) content. The methodfor generating a reducing gas having a variable combustibles content mayalso include providing a hydrocarbon fuel stream; combining thehydrocarbon fuel stream and the oxidant stream into a feed mixture, thefeed mixture having an oxidant/fuel ratio; and reacting the feed mixturein a catalytic reactor to generate the reducing gas. The method forgenerating a reducing gas having a variable combustibles content mayalso include controlling both the O₂ content of the oxidant stream andthe oxidant/fuel ratio of the feed mixture to maintain a predeterminedcontrol temperature while also independently varying both the O₂ contentof the oxidant stream and the oxidant/fuel ratio of the feed mixture toobtain a selected combustibles content of the reducing gas.

One refinement of the embodiment may include supplying the reducing gasto a fuel cell.

Another refinement of the embodiment may include generating anitrogen-rich stream; and combining the nitrogen-rich stream with air toyield the nitrogen-rich oxidant stream.

Another refinement of the embodiment may include the nitrogen-richoxidant stream generated by extracting nitrogen from air using anitrogen separation membrane.

Another refinement of the embodiment may include the predeterminedtemperature a predetermined temperature of the reducing gas.

Another refinement of the embodiment may include the predeterminedtemperature a predetermined temperature of a catalyst of the catalyticreactor.

Another refinement of the embodiment may include providing feedbackinformation pertaining to at least two of the O₂ content, thetemperature of the reducing gas and at least one of a hydrogen (H₂)content of the reducing gas and a carbon monoxide (CO) content of thereducing gas. The controlling both the O₂ content of the oxidant streamand the oxidant/fuel ratio of the feed mixture to maintain thepredetermined temperature is performed based on the feedbackinformation.

Another embodiment of the present invention may include a method forgenerating a reducing gas and using the reducing gas in a fuel cell. Themethod for generating a reducing gas and using the reducing gas in afuel cell may include a step for providing an oxidant stream; a step forsupplying a hydrocarbon fuel; a step for combining the oxidant and saidhydrocarbon fuel into a feed mixture, the feed mixture having anoxidant/fuel ratio; and a step for catalytically converting the feedmixture to generate a reducing gas. The method for generating a reducinggas and using the reducing gas in a fuel cell may also include a stepfor controlling both the O₂ content of the oxidant stream and theoxidant/fuel ratio of the feed mixture to maintain a predeterminedtemperature of the reducing gas while also independently varying atleast one of the O₂ content of the oxidant stream and the oxidant/fuelratio of the feed mixture to obtain a selected combustibles content ofthe reducing gas; and a step for directing the reducing gas to the fuelcell.

Another embodiment may include a method for shutting down a fuel cell,including: receiving a command to shut down the fuel cell from the powerproduction mode; receiving air from an air intake; generating anitrogen-rich gas stream by extracting oxygen (O₂) from the air receivedfrom the air intake; initiating flow of a hydrocarbon fuel stream inresponse to the command; combining the hydrocarbon fuel stream with thenitrogen-rich gas stream to yield a feed mixture; directing the feedmixture to the at least one of a heater, a catalyst and an inlet to thecatalyst, wherein the at least one of the heater, catalyst and inlet tothe catalyst are heated to a preheat temperature selected for catalyticauto-ignition of the feed mixture; converting the feed mixture into areducing gas using the catalyst; and directing the reducing gas to atleast one of an anode and a reformer of the fuel cell.

In a refinement, the method may include maintaining the at least one ofthe heater, the catalyst and the inlet to the catalyst at the preheattemperature selected for catalytic auto-ignition of the feed mixtureduring fuel cell operation in power production mode prior to receivingthe command to shut down the fuel cell.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment(s), but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as permitted under the law. Furthermore itshould be understood that while the use of the word preferable,preferably, or preferred in the description above indicates that featureso described may be more desirable, it nonetheless may not be necessaryand any embodiment lacking the same may be contemplated as within thescope of the invention, that scope being defined by the claims thatfollow. In reading the claims it is intended that when words such as“a,” “an,” “at least one” and “at least a portion” are used, there is nointention to limit the claim to only one item unless specifically statedto the contrary in the claim. Further, when the language “at least aportion” and/or “a portion” is used the item may include a portionand/or the entire item unless specifically stated to the contrary.

What is claimed is:
 1. A method for generating a reducing gas having avariable combustibles content, comprising: providing a nitrogen-richoxidant stream, the oxidant stream having a nitrogen content greaterthan that of ambient air, the oxidant stream also having an oxygen (O₂)content; providing a hydrocarbon fuel stream; combining the hydrocarbonfuel stream and the oxidant stream into a feed mixture, the feed mixturehaving an oxidant/fuel ratio; reacting the feed mixture in a catalyticreactor to generate the reducing gas; and controlling both the O₂content of the oxidant stream and the oxidant/fuel ratio of the feedmixture to maintain a predetermined control temperature while alsoindependently varying both the O₂ content of the oxidant stream and theoxidant/fuel ratio of the feed mixture to obtain a selected combustiblescontent of the reducing gas.
 2. The method of claim 1, furthercomprising supplying the reducing gas to a fuel cell.
 3. The method ofclaim 1, further comprising: generating a nitrogen-rich stream; andcombining the nitrogen-rich stream with air to yield the nitrogen-richoxidant stream.
 4. The method of claim 1, wherein the nitrogen-richoxidant stream is generated by extracting nitrogen from air using anitrogen separation membrane.
 5. The method of claim 1, wherein thepredetermined temperature is a predetermined temperature of the reducinggas.
 6. The method of claim 1, wherein the predetermined temperature isa predetermined temperature of a catalyst of the catalytic reactor. 7.The method of claim 1, further comprising: providing feedbackinformation pertaining to at least two of the O₂ content, thetemperature of the reducing gas and at least one of a hydrogen (H₂)content of the reducing gas and a carbon monoxide (CO) content of thereducing gas, wherein said controlling both the O₂ content of theoxidant stream and the oxidant/fuel ratio of the feed mixture tomaintain the predetermined temperature is performed based on thefeedback information.
 8. A method for generating a reducing gas andusing the reducing gas in a fuel cell, comprising: a step for providingan oxidant stream; a step for supplying a hydrocarbon fuel; a step forcombining the oxidant and said hydrocarbon fuel into a feed mixture, thefeed mixture having an oxidant/fuel ratio; a step for catalyticallyconverting the feed mixture to generate a reducing gas; a step forcontrolling both the O₂ content of the oxidant stream and theoxidant/fuel ratio of the feed mixture to maintain a predeterminedtemperature of the reducing gas while also independently varying atleast one of the O₂ content of the oxidant stream and the oxidant/fuelratio of the feed mixture to obtain a selected combustibles content ofthe reducing gas; and a step for directing the reducing gas to the fuelcell.
 9. A method for generating a reducing gas having a variablecombustibles content, comprising: providing a nitrogen-rich oxidantstream, the oxidant stream having a nitrogen content greater than thatof ambient air, the oxidant stream also having an oxygen (O₂) content,wherein the nitrogen-rich oxidant stream is generated by extractingnitrogen from air using a nitrogen separation membrane; providing ahydrocarbon fuel stream; combining the hydrocarbon fuel stream and theoxidant stream into a feed mixture, the feed mixture having anoxidant/fuel ratio; reacting the feed mixture in a catalytic reactor togenerate the reducing gas; and controlling both the O₂ content of theoxidant stream and the oxidant/fuel ratio of the feed mixture tomaintain a predetermined control temperature while also independentlyvarying both the O₂ content of the oxidant stream and the oxidant/fuelratio of the feed mixture to obtain a selected combustibles content ofthe reducing gas.